The present invention relates to a touch probe used, for example, on a coordinate positioning machine such as a machine tool or coordinate measuring machine.
Such probes are known from U.S. Pat. No. 4,153,998, and comprise a fixed structure by which the probe may be connected to the movable arm of the machine, relative to which a stylus having a measuring tip for contacting the workpiece is supported. The position of a workpiece surface is detected by operating the machine to drive the measuring tip of the stylus into contact with the surface whose position is to be measured, and detecting, with the probe, contact between the measuring tip and the surface. The probe emits a signal indicative of such contact, and this is used to record the position of the movable arm and to instruct the machine control to brake its movement. In order to prevent damage to the stylus when, during braking of its movement, the movable arm overtravels the point of contact between the stylus tip and surface, the stylus is carried on a stylus-supporting member which is supported on the fixed structure in a rest location, out of which it may be displaced when a deflecting force is applied to the stylus, and to which it may return when the deflecting force is removed. This displacement of the stylus- supporting member is known as "overtravel displacement". Good positional repeatability of the stylus-supporting member relative to the fixed structure from one overtravel displacement to the next is required in order that the position of the surface may be measured accurately.
It is, on occasions, desirable to change the stylus carried by the probe. For example, the measurement of different features on a workpiece may require the use of different geometries and lengths of styli.
A system for changing styli is known from U.S. Pat. No. 4,649,623 and comprises a retaining member on the stylus-supporting member, on which a stylus selected from a magazine containing a number of different styli is magnetically retained. One stylus may be exchanged for another stylus in the magazine by using an electro-magnet to neutralise the magnetic attraction forces between the retaining member and the stylus carried on the retaining member. Mutually engageable engagement elements on the retaining member and each stylus provide kinematic location of a given stylus on the retaining member, and thus good repeatability of the position of the measuring tip of each stylus relative to the retaining member from one stylus change operation to the next.
However, a fundamental problem with such systems is that mechanical hysteresis increases with the number of joints or support mechanisms which perform the function of locating the measuring tip relative to the fixed structure. Thus, for example, in the probe of U.S. Pat. No. 4,649,623 the stylus-supporting member is supported within the fixed structure in a kinematic rest position which provides overtravel displacement of the stylus during the course of a measuring operation, and a given stylus is supported on the stylus-supporting member in a further kinematic location which is broken and reformed when a stylus is removed and replaced during the course of an inspection cycle.
A first aspect of the present invention provides a system for changing styli on a probe in which a given stylus is interchangeably magnetically retained on a retaining member supported on the fixed structure of the probe, such that a single mechanical location between a given stylus and the retaining member performs both the function of locating the stylus on the retaining member during a stylus change operation, and providing overtravel displacement of the stylus during a measuring operation.
Accordingly, the present invention provides a touch probe for use on a coordinate positioning machine, comprising:
a fixed structure by which the probe may be supported on a movable arm of the machine; PA1 a retaining member supported on the fixed structure; PA1 a stylus having a supporting body, a stem extending from the supporting body, and a sensing tip provided at a free end of the stem; PA1 means for releasably locating said supporting body in a repeatable rest position on said retaining member including magnetic biasing means for biasing said supporting body into said rest position, to enable exchange of one such stylus for another; wherein PA1 said releasable location means enables tilting displacement of said supporting body (and thus said stylus) out of said rest position in response to a displacing force on the stylus, and return of said supporting body to said rest position, under the influence of said magnetic biasing means.
To provide a truly flexible measuring system, a plurality of styli must be retained within the working area of the machine to enable automatic exchange of one stylus for another. A second independent aspect of the present invention relates to a storage system for retaining a plurality of task modules (such as a stylus as defined above) on a machine, wherein each of the task modules is interchangeably connectable to a retaining module (such as the fixed structure and retaining member) supported on the movable arm of the machine.
According to a second aspect of the present invention, a magazine is provided which includes a plurality of storage ports, each for supporting a task module, each said task module having a set of engagement elements, magnetising means being provided for urging said task module in a first direction into contact with said storage port, wherein said storage port and said task module are configured so that said engagement elements are accessible from a direction opposite to said first direction, and that, upon application to said task module of a force exceeding the force from the magnetising means, said task module may move relative to said support structure in said direction opposite to said first direction.
Such a magazine and task modules enable engagement of a task module by a retaining module, and disengagement of the task module from the storage port in a single continuous movement and without any additional machine apparatus (such as dedicated motors or electromagnets).
Typically a task module may be the supporting body, stem, and sensing tip of a stylus. However more complex forms of task module may be provided (e.g. a temperature sensor, an accelerometer or a pressure sensor) with, where required, an appropriate retaining module.
A further type of task module is disclosed in our copending European Patent Application No. EP-A-501,710. This application discloses a touch probe which has a task module in the form of a stylus module and a retaining module provided by a strain sensitive module having a load cell upon which the stylus module is interchangeably retainined. The stylus module has a casing, and a stylus supported relative to the casing in such a way as to enable overtravel displacement of the stylus relative to the casing. In this probe, contact between the sensing tip of the stylus and a workpiece is detected by sensing strain in the stylus (which occurs before overtravel displacement of the stylus supporting member) transmitted through the casing of the stylus module to the strain sensitive load cell. When this strain reaches a predetermined threshold electronic circuitry associated with the load cell causes the emission of a trigger signal to the machine control.
We have discovered however that an associated disadvantage exists with this modular construction of probe. If, due to an error in the part inspection program of the machine, the probe is driven so that the stylus module accidentally becomes detached from the strain sensitive retaining module (e.g. due to excessive overtravel), the probe no longer has the capacity to register a trigger without crashing the retaining module into a surface, and thus causing irreparable damage. In spite of this, the control of the machine will continue to operate the machine as if the probe is completely functional; evidently, this situation may result in serious damage to either the machine or the probe or both.
A third independent aspect of the present invention provides a probe including a retaining module and a stylus module which is releasably mountable to the retaining module, wherein the retaining module comprises means for detecting when a stylus module is mounted thereto, and for outputting an alarm signal accordingly.
The detecting means may be provided by any suitable mechanism e.g. a microswitch, an optical sensor or a magnetic sensor.
The alarm signal from the detecting means is used to instruct the machine control to arrest motion of the movable arm on which the retaining module is supported, and thus prevent any damage to either the machine or the retaining module. However, the generation of this alarm signal gives rise to a further problem in that, during a stylus changing operation, the machine is operated to deposit a stylus module in a vacant storage port of the magazine, and then to move the retaining module to a different storage port in order to engage a new stylus module. Inevitably, the retaining module does not carry a stylus module while it moves between two ports of the magazine in the course of a stylus changing operation; the detecting means will thus emit an alarm signal preventing movement, and effectively prevent automatic stylus changing.
A fourth aspect of the present invention relates to a magazine having means for sensing when a retaining module is in close proximity, and for outputting an inhibit signal (which overrides the effect of the alarm signal).
The machine may thus drive the movable arm during stylus changing, even though a stylus module is not mounted to the retaining module. Preferably the sensing means is provided on the magazine, and may e.g. be one or more light beams and/or a sensor for sensing electric or magnetic fields, such as a Hall Effect sensor.
The magazine is preferably mounted on a base supported on the machine, and is biased into a repeatable rest position with respect thereto, wherein further detecting means are provided for detecting deflection of the magazine relative to the base and for outputting a crash signal accordingly. Such deflection may be caused by accidental collision between a retaining module and the magazine during a stylus changing operation.